Theoretical and numerical study on uniaxial compressive behaviour of marl

Rock matrix failure modes and the consequent material strength are related to the testing conditions, such as the friction value at the rock press contact and the testing velocity, and to the shape and volume of the rock specimen. However, when differences are observed even in specimens having the same rock types in similar boundary conditions, the presence of microscopic cracks is assumed even when they are not directly observable. Porosity is a constant natural presence in rocks with different percentages and shapes whilst cracks can also be induced by rock sampling procedures and by specimen preparation. Defects of both natures (pores and cracks) influence the failure mode and the consequent mechanical features. However, the relation between failure mode and rock strength is not clear yet. This work is aimed to analyze the connection among several aspects influencing rock failure modes by means of laboratory testing in uniaxial condition and tridimensional numerical modeling. The study concerns a French marl chosen for its homogeneity and uniformity when compared to other marls. Specimens with different shapes and volumes were tested with two different laboratory devices equipped with different displacement measuring systems. Numerical modeling reproducing laboratory testing was carried out with the aim of gathering further information concerning the rock stress and strain state during loading. Moreover, the influence of different friction values between the rocks and the press and different rock porosity values have also been considered.